Gas turbine engines operate at high temperatures. Differential thermal expansion of components can influence the dimension of a clearance space between the tip of a turbine blade and a shroud. Leakage between the tip of a turbine blade and its shroud can result in a significant reduction in the turbine's efficiency. Consequences of contact between the blade tip and shroud can be life limiting for the components. There is a desire to maintain an optimum clearance space between the blade tip and shroud during the various operational stages of a gas turbine engine.
In some prior known arrangements, radial expansion of the shroud may be restricted by the presence of a radially outer turbine casing. The casing may connect, through radial struts, to segments of the shroud. Thermal expansion and contraction of the turbine casing may be controlled by the introduction of an air supply at a temperature which encourages a desired amount of thermal expansion or contraction of the casing when targeted at the casing from a radially outer side. One example of such an arrangement is known from the Applicant's own prior published U.S. Pat. No. 6,863,495 B2.
FIG. 1 illustrates schematically a prior known arrangement for cooling the casing of a turbine in a gas turbine engine. As shown in the Figure, a manifold 1 is connected to a supply of air via a variable control valve 2. For example, the supply of air is taken from the compressor. When the valve is opened, the air flows around the manifold 1 and into a number of segments 3 which are radially spaced from but in thermal communication with a turbine casing 4. The casing 4 may be connected to a turbine shroud (not shown) sitting within the casing 4 by means of one or more radially extending struts (not shown). Each segment 3 is provided with a plurality of impingement cooling holes 5 through which the air from the manifold 1 is directed at a radially outer wall of the casing 4. With the impinging air at a temperature different to that of the casing 4 thermal resizing of the casing 4 results. By means of the struts, segments of the shroud can be repositioned along a radius to accommodate thermal resizing of turbine blades enclosed by the shroud. The valve 2 is a two stop valve configurable to deliver a low flow during maximum take-off (MTO) operation of the gas turbine engine and a higher flow during a cruise operation of the gas turbine engine. The impingement cooling holes 5 serve to meter and restrict the higher flow delivery to the casing 4. It will be appreciated that the rate of flow of the air to the casing will influence the rate at and extent to which the casing can be caused to undergo thermal resizing.